As Trillian says, at that age you can often get quite small gains with no problem. My DS was very active and crawling at that age so his gains slowed right down. He went from the 90th percentile (he had huge gains in the first 3 months) to the 25th percentile. The MCHN was never worried though as he was clearly getting enough milk and healthy. I wouldn't think that 300g in a month at that age would be anything to worry about unless there were other signs. Sometimes health professionals can get a bit too focused on numbers rather than the bigger picture.